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    <a href="SP_0_Main.htm"><img src="images/abacus.gif" border="0" width="32" height="32" alt="Calc." title="Calc."></a>&nbsp;</td>
    <td align="left" valign="middle"><font size="-1"><a href="SP_0_Main.htm">SED &amp; PREDOM</a>&nbsp;/ <a href="SP_Ionic_Strength.htm">Ionic&nbsp;Strength</a></font></td>
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<p><i>Activity</i> corresponds to the concentration for a solution behaving ideally.
It has the same units as concentration.</p>

<p>The activity coefficient, <nobr><i>&#947;<sub>i</sub></i>,</nobr> gives the
relationship between concentration and activity:</p>

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<nobr><i>&#947;<sub>i</sub></i> = <i>a<sub>i</sub></i> / <i>m<sub>i</sub></i> </nobr>
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where <nobr><i>a<sub>i</sub></i></nobr> and and <nobr><i>m<sub>i</sub></i></nobr> are the
activity and <a href="SP_Molality.htm">molality</a>,
respectively, of the aqueous <nobr>species &#147;<i>i</i>&#148;.</nobr></p>

<p>In aqueous solutions the activity coefficients depend on the
<a href="SP_Ionic_Strength.htm">ionic strength</a>.</p>

<p>When writing equations, concentrations are usually written with brackets,
for example <nobr>&#147;[H<sup>+</sup>]&#148;,</nobr> while activities are
often written with braces, <nobr>&#147;{H<sup>+</sup>}&#148;.</nobr></p>

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